If引导的从句与将来的事实相反

If 用将来时，主句用过去将来时。

  过去将来时

如果将来我要出国，我会去美国。

If I were to go abroad, I would go to America.

Were I to go abroad,  I would go to America.

If I should go abroad, I would go to America.

ShouldI to go abroad, I would go to America.

如果我们能活到200岁，我们就能改变所有事情。

If we were to be 200 years old, we could change everything.

如果你去看玛丽的话，你应该告诉她什么？

If you were to see Mary, what should you tell her?

如果if从句中有were、had、should，可以把if省略，把were、had、should提前到句首。